Modern fiber optic transmission systems are based on a polarization-multiplexed, wavelength-division multiplex architecture thus taking advantage of all four degrees of freedom of the optical field. The optical field is, from a mathematical perspective, a complex quantity having real and imaginary components (sometimes referred to as in-phase and quadrature components (IQ), respectively). The two optical field components are mutually orthogonal, which in mathematical terms, signifies extreme dissimilarity, or independence. In addition to the complex characteristics of the optical field, the optical field can exist in two orthogonal polarizations, that, too, are altogether dissimilar. Thus, an ideal optical field has four orthogonal components in total.
Ideally, sending information imprinted (e.g. modulated) onto the four orthogonal components of an optical field is equivalent to sending information over four independent channels, thus providing means for increasing the density of information that can be transmitted by an optical field by a four-fold.
Unfortunately, the (ideally) independent information channels of an optical field often mix, or couple, with each other due to loss of polarization orthogonality and imbalances in power between the polarizations in the modulation process. This results in degraded performance of the optical transmission system.
In practice, strict orthogonality between the polarizations is difficult to realize—especially orthogonality of the polarization in components used for modulation of the optical field. One cause of the loss of orthogonality between the polarizations includes crosstalk between optical modulator electrodes used for modulating the respective polarizations, as crosstalk introduces similarities across the polarizations.
Equal power balance between the two polarizations is also difficult to realize. This is especially problematic as modern optical transmission systems are designed to transmit (“launch”) using equal power in the two polarizations, and digital processing methods for later polarization de-coupling at a receiver often rely on the assumption of equal power launch for each of the two polarizations. In practice, the power in the two polarizations, even at the transmitter, is rarely equal due to the unequal power split between the polarization modulation branches, or possibly polarization dependent loss of the involved components. The latter combination of effects leads to an irreversible penalty, which, owing to the nonlinear response of the modulator and possibly other elements in the system, cannot be reversed (or undone) at the receiving end of the link. Consequently, owing to the non-ideal characteristics of the components in the realization, optical dual-polarization transmission systems do not operate to their full potential. Specifically, degradation by more than 3 dB is a common occurrence in these systems.
Some methods attempt to correct the polarization signals at the receiving end of the transmission link, but still suffer a significant penalty with respect to theoretical predictions (assuming signal modulation in orthogonal polarizations). Other solutions attempt to improve signal generation quality at the transmitter by attempting to equalize (adjust) each of the four modulation streams independently during the modulation process by some means of pre-compensation, but these solutions still have a residual error in attainable fidelity.